Polysiloxane copolymers and networks are desirable because they possess a variety of unique and superior properties. For example, polysiloxane films and coatings provide low energy surfaces associated with the following properties: low coefficients of friction, longer wear life, biocompatibility, good releasing properties from adhesive surfaces and/or mold surfaces, and good antiblocking and lubricating properties usable for textile coatings and fiber spinning resins. Polysiloxane sealants and adhesives also exhibit resistance to degradation by ultraviolet radiation, have excellent flexibility at low temperatures while retaining stability at high temperature, are impervious to water and may be cured by convenient and economical methods.
However, a disadvantage to presently available crosslinked polysiloxanes for the above applications is their relatively poor mechanical strength. The low glass transition temperature coupled with the low intermolecular forces of presently available crosslinked polysiloxanes, provide no mechanism for the attainment of mechanical strength. On the other hand, polyurethane elastomers possess excellent mechanical properties, associated with their high degree of hydrogen bonding.
It would thus be desirable to provide a class of polysiloxanes retaining the desirable properties of crosslinked polysiloxane elastomers with improved mechanical properties.
The present invention provides tetrafunctional hydroxyl polysiloxane oligomers, having the desirable properties of polysiloxanes, which, however, can be cured with diisocyanates to form hydrolytically stable polysiloxane networks with improved mechanical properties. At least part of this improved mechanical strength is attributable to the hydrogen bonding interactions associated with the urethane bond. Furthermore, no volatile products are involved in the curing reaction.
The present invention further provides a class of difunctional hydroxyl polysiloxane oligomers which are useful to produce block copolymers for such materials as, for example, polysiloxane containing polyurethanes, polyesters, polycarbonates, and polysulfones. Although both the difunctional and tetrafunctional hydroxyl polysiloxane oligomers of the invention may form networks if reacted with multifunctional chain extenders, the tetrafunctional polysiloxanes are preferable for this purpose.
The difunctional hydroxy polysiloxane oligomers of the invention are useful for the production of polysiloxane/polycaprolactone block copolymers and for the synthesis of polysiloxane containing polyurethanes, polycarbonates, polyesters, polysulfones. The difunctional hydroxy polysiloxane copolymers are particularly useful in cases where both the properties of a hard block (a polyurethane block, a polycarbonate block, and the like) are needed and the properties of polysiloxane are needed. Among such polysiloxane properties are low energy surfaces, biocompatible polymers, high gas permeabilities, resistance to plasma etching, good UV stability, low temperature flexibility, thermally stable elastomers, etc., coupled with excellent mechanical properties.
The difunctional hydroxy polysiloxane oligomers are further useful for the polymerization of ring-opening monomers which would be initiated by hydroxyl groups or derivatives of hydroxyl groups. This would include initiation of lactones, epoxy groups, anhydride/epoxy mixtures. Examples of lactones which may be initiated for polymerization include epsilon-caprolactone, epsilon-methyl caprolactone, delta-valerolactone. Examples of epoxides include ethylene oxide, propylene oxide, styrene oxide, epichlorohydrin, allyl glycidyl ether. Examples of anhydride/epoxy mixtures include maleic anhydride/ethylene oxide, phthalic anhydride/maleic anhydride/propylene oxide, and phthalic anhydride/maleic anhydride/ethylene oxide.
It is believed that polysiloxanes having improved mechanical strength have not been heretofore formed or taught by the prior art because of at least the following two problems. Firstly, heretofore the molecular weight of the hydroxyl terminated polysiloxane blocks wherein the hydroxyl groups are bonded to the silicon via hydrolytically stable Si--C bonds were not facilely controlled. Since mechanical properties are dependent upon the block lengths in the networks, the ability to control the block lengths is important. Secondly, while a hydroxyl-terminated polysiloxane might be desirable for forming a polysiloxane network, the art taught that terminal hydroxyl groups, in the presence of strong acids used for forming polysiloxanes by redistribution reactions, will dehydrate (See J. Polym. Sci., A-1, 4, 2325 (1966)). Consistent with our results obtained using hydroxybutyl terminated siloxane dimers and oligomers in the presence of strong acid catalysts, Speier, et al. (J. L. Speier, M. P. David, and B. A. Eynon, J.O.C., 25, 1637 (1960)) describe the dehydration mechanism of 1,3-bis(hydroxypropyl)tetramethyldisiloxane as cyclization by attack of the hydroxyl group on the terminal silicon atom. This backbiting reaction thus removes the functional end groups in the equilibration reactions from availability for further reaction, therefore making the molecular weights and functionalities of the resultant polysiloxane oligomers uncontrollable.
The present invention provides hydroxyl-functional polysiloxane oligomers with the hydroxyl groups attached to the terminal silicon atoms via hydrolytically stable Si-C bonds which can be formed in a way to control their molecular weight to desired parameters, without the problem of the backbiting reaction.
It is therefore an object of the present invention to provide hydroxyl-functional disiloxanes consistent with the above described structures which may be expanded by siloxane redistribution reactions to hydroxyl-functional polysiloxane oligomers.
It is another object of the present invention to provide the difunctional hydroxyl polysiloxane oligomers produced in a controlled manner to desired molecular weights, which are useful to synthesize linear, siloxane-containing, thermoplastic and elastomeric block copolymers.
It is a further object of the present invention to provide novel hydroxyl-functional polysiloxane oligomers which may be networked into polysiloxane compositions having improved mechanical strength.
It is a further object of the present invention to provide a method for forming these polysiloxane oligomers having hydroxy-functional groups using acidic or basic conditions without encountering th problem of the backbiting reaction.
Additional objects, advantages and novel features of the present invention will be set forth in part in the following description and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.